In the prior art closed-loop emission control apparatus, the concentration of an exhaust composition such as residual oxygen is sensed and fed back to an air-fuel mixing and proportioning device to control the air-fuel ratio of the mixture delivered to the engine. The noxious components (CO, HC and NOx) are simultaneously converted into harmless products at the maximum efficiency if the air-fuel ratio is controlled at a value in the vicinity of stoichiometry. The exhaust composition sensor such as oxygen sensor is usually operable at elevated temperatures higher than 400.degree. C., and during engine warm-up periods the output from the composition sensor remains at a low voltage level. Under these circumstances, it is desirable to inhibit the feedback operation and allow the engine to operate in open-loop mode using the normal carburetion or fuel injection. Since the operating characteristic of the oxygen sensor is such that its output has a steep transition in amplitude at stoichiometry from the high voltage state for richer mixtures to the low voltage state for leaner mixtures, when the sensor's operating temperature has been reached and if leaner mixtures are supplied under the open-loop mode, then the sensor output still continues its low voltage condition and will cause the system to hangover in the open-loop mode even though the temperature condition warrants feedback control. Therefore, it is necessary to supply a quantity of rich mixtures prior to the start of closed-loop feedback control in order to reduce the hangover time. Actually, the components that make up a control loop are manufactured with a different degree of accuracy, so that the total value of accuracy of the loop of electronic fuel injection, for example, may amount to .+-.10%. With electronic fuel injection in which the width of the injection pulse is controlled by a signal containing proportional, integral and DC bias components, a 10% increase of the DC bias under open-loop mode would amount to a 20% increase in fuel supply in an extreme case. This is unfavorable from the emission control standpoint.